For disposable products, it is desirable to use materials which are both biodegradable and water-dispersive.
Biodegradable polymers disposed of in bioactive environments degrade by the enzymatic action of microorganisms such as bacteria, fungi and algae. Their polymer chains may also be cleaved by non-enzymatic processes such as chemical hydrolysis. As used herein, the term “biodegradable” means that the composition degrades within one year using the standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions.
As used herein, the term “water-dispersible” means that the composition dissolves or breaks into pieces smaller than 0.841 mm (20 mesh) after being immersed in water for approximately 24 hours at room temperature.
Poly(lactic acid) or polylactide (PLA), is an attractive biodegradable and biocompatible polymer. It is derived from renewable resources (e.g., corn, wheat, or rice) and it is biodegradable, recyclable, and compostable. In addition, PLA exhibits excellent processability. Actually, PLA has better thermal processability compared to other biodegradable such as poly(hydroxyalkanoates) (PHAs), poly(ε-caprolactone) (PCL), etc. It can be processed by injection moulding, film extrusion, blow moulding, thermoforming, fibre spinning, and film forming. However, the use of PLA can be restricted due to the fact that it is a hydrophobic polymer and is not able to solubilise or disperse in water.
Water-soluble biodegradable polymers may be synthesized by modifying starch and cellulose. For example, carboxymethyl cellulose (CMC) having different degrees of carboxymethyl substitution is a family of marketed water-soluble polymers. Hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), methyl cellulose (MC) and ethyl cellulose (EC) are used as binders, water retention aids, thickeners, film formers, lubricants, or rheology modifiers. Water-soluble polysaccharides are also produced by microbial fermentation. Xanthan is the most widely used microbial polysaccharide. Industrial uses of xanthan include oil recovery (viscosity control), paper manufacturing, agriculture (stimulation of plant growth), and cosmetics. Pullulan has also shown various potential applications. For example, its good moisture retention and low oxygen permeability has led to its use as edible films for food packaging.
To date, poly(vinyl alcohol) (PVOH) is the only polymer with exclusively carbon atoms in the main chain that is regarded as biodegradable. It is currently used in textiles, paper and packaging industries as paper coatings, adhesives, and films. Importantly, PVOH is water soluble.
A disadvantage associated with water-soluble biodegradable polymers is that they are unduly sensitive to water, which limits their use for most of the conventional polymer applications. It is therefore desirable to provide a material that may be used in the manufacture of disposable articles and which is water-responsive. Preferably, such material should be versatile and inexpensive to produce. It is also desirable for the material to be stable enough for defined applications but subject to degradation under predetermined conditions.
The use of polymers for the production of water-dispersive articles is known in the art. Mainly, compositions comprising multilayer polymer films are reported. Indeed, there are many examples of multilayer films that are utilized in disposable objects. Most of these examples consist of films or fibres which comprise external layers of an environmentally degradable polymer and an internal layer of a water-responsive polymer. The utility of such structures lies in the adjustment of physical properties in order to increase the stability or lifetime of such structure. For example U.S. Pat. No. 4,826,493 describes the use of a thin layer of hydroxybutyrate polymer as a component of a multilayer structure as a barrier film.
Another example of the use of multilayer films is U.S. Pat. No. 4,620,999, which describes the use of a water soluble film coated with, or laminated to, a water insoluble film as a disposable bag. A similar type of bag is disclosed in JP 61-42127. It is composed of an inner layer of water-resistant water-dispersible resin such as polylactide and an outer layer of polyvinyl alcohol. These examples, however, are all limited to compositions consisting of layers of different polymers, and do not encompass actual blends of different polymers.
Other water-responsive articles are disclosed in U.S. Pat. No. 5,508,101, U.S. Pat. No. 5,567,510, and U.S. Pat. No. 5,472,518. These patents disclose a series of water-responsive compositions comprising a hydrolytically degradable polymer and a water soluble polymer. The articles are, however, constructed from polymers which are first formed into fibres or films and then combined. Therefore, although the fibres and films of the polymers of such compositions are in very close proximity, they are not actual blends.
Polymer blending is an attractive approach to the tailoring of the properties of polymeric materials without having to invest in new chemistry. Among the different blend morphologies, co-continuous polymer blends exhibit the best performance improvements because both components can fully contribute to the properties of the blend. However, poor interface between the different polymeric phases of the blend usually leads to a significant loss of properties and, more specifically, a deterioration in mechanical performance is observed. To overcome this problem, traditionally compatibilizers are used to strengthen the interface. In this field, the technique of reactive compatibilization is a very attractive and economical route to achieve stable, multiphase polymer blends.
For most binary polymer blends, the appropriate reactive groups are not present and functionalisation of the blend components is required. However, for some binary polymer blends, a reactive polymer can be added as compatibilizer precursor which is miscible with one of the blend components and reactive towards the other. This type of blend compatibilization can be advantageously achieved by reactive extrusion.
Reactive extrusion (REX) is a polymer processing technique that mainly involves the use of an extruder as a chemical reactor. Polymerization and other chemical reactions such as reactive compatibilization are carried out in situ, while processing is in progress. Therefore, REX differs from conventional polymer manufacturing methods, where synthesis is a separate operation and the extruder serves only as a processing aid.
U.S. Pat. No. 5,945,480 discloses components of flushable personal care products manufactured using fibres based on blends of polyvinyl alcohol and polylactide. The blend components are compatibilized by means of polylactide modified with 2-hydroxyethyl methacrylate (HEMA). Although the disclosed invention aims to improve the compatibility of the blended polymers, there is no reference to reactive compatibilization. Actually, only hydrogen bonding between hydroxyl groups of HEMA and polyvinyl alcohol can be expected. The examples, which do not describe the formation of any chemical covalent bond for promoting blends compatibility, do not correspond to actual reactive compatibilization procedures.
Polymer blend compositions for making fibres and films that are optimally combined are desirable because they are highly stable. Optimal combination of polymers means that the polymer interface is improved in such a way that the polymer blends exhibit co-continuous morphology. This may be achieved by means of reactive extrusion. Tailored blend properties can be obtained by the judicious choice of reactive compatibilizers. Although blended polymer compositions are known, reactively compatibilized co-continuous polymer blends are desirable since the resulting composition is more stable and versatile.
In light of the foregoing, it is desirable to produce biodegradable and water-dispersive polymer blends, preferably which can be easily processed to produce films and fibres. It is also desirable to provide thermally processable polymer blends which have good mechanical and physical properties.